The invention provides a process for producing polycrystalline silicon.
Polycrystalline silicon (polysilicon for short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (float zone or FZ process). This monocrystalline silicon is divided into wafers and, after a multitude of mechanical, chemical and chemomechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is being required to an increased degree for production of mono- or polycrystalline silicon by means of pulling or casting processes, and this mono- or polycrystalline silicon serves for manufacture of solar cells for photovoltaics.
Polycrystalline silicon, often also referred to as polysilicon for short, is typically produced by means of the Siemens process. This involves heating thin filament rods of silicon by direct passage of current in a bell-shaped reactor (“Siemens reactor”), and introducing a reaction gas comprising a silicon-containing component and hydrogen.
In addition, it is also known that small silicon particles in a fluidized bed reactor can be exposed directly to such a reaction gas. The polycrystalline silicon thus obtained is in the form of granules (granular poly).
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane, more preferably trichlorosilane. Predominantly, SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
In the Siemens process, the filament rods are typically vertical in electrodes present at the reactor base, through which they are connected to the power supply. In each case two filament rods are coupled via a horizontal bridge (likewise composed of silicon) and form a carrier body for the silicon deposition. The bridge coupling produces the typical U shape of the carrier bodies, which are also called thin rods.
On the heated rods and the bridge, high-purity silicon is deposited, as a result of which the rod diameter grows with time (CVD=chemical vapor deposition/gas phase deposition).
A first aim is to produce polysilicon at minimum expense. However, the quality demands on the part of the customer are also increasing. Thus, a second aim is to minimize the proportion of extraneous atoms, for example carbon and dopants, in the polycrystalline silicon. The purity of the starting materials, such as chlorosilane and hydrogen, plays a crucial role here. The prior art firstly attempts to purify the starting materials before the deposition and to protect them from new impurities in all subsequent processes.
TCS is produced typically by reaction of metallurgical silicon with HCl in a fluidized bed reactor. It has been found that the purity of TCS can be increased by means of distillation; cf. Lee P. Hunt in “Handbook of semiconductor technology”, edited by O'Mara, Herring and Hunt, ISBN 0-8155-1237-6, page 4, FIG. 2). This is based on the fact that the boiling point of TCS is approx. 32° C. (at standard pressure) and thus differs considerably from the boiling points of most unwanted impurities and by-products, for example dichlorosilane.
It is additionally known that substances obtained as offgas after the deposition, such as silicon tetrachloride (STC) and HCl, and also unreacted TCS and hydrogen, can be separated and purified, and then TCS and hydrogen can be supplied together with new TCS and hydrogen back to the deposition; cf. Leo C. Rogers in “Handbook of Semiconductor technology”, edited by O'Mara, Herring and Hunt, ISBN 0-8155-1237-6, page 56, FIG. 6.
However, distillation processes cannot solve all problems since unwanted substances such as isopentane have a similar boiling point to TCS. Thus, sufficient separation of the substances from TCS is impossible.
EP 2 033 937 A2 describes a process which binds isopentane with chlorine in order thus to be able to better separate it from TCS by means of fractional distillation.
EP 2 036 858 A2 describes the conversion of boron- and phosphorus-containing impurities with small amounts of oxygen and aromatic aldehydes in order to increase the boiling points of the boron- and phosphorus-containing substances. Subsequently, separation is effected by means of fractional distillation.
DE 1 667 742 A1 discloses a process for purifying TCS by means of distillation, wherein a distillation temperature only insignificantly higher than the boiling point of TCS is used.
In the deposition of silicon too, there are known measures for avoiding unwanted impurities in silicon.
In DE 1 222 481 B, the offgas of a first deposition reactor is then passed directly into a second deposition reactor. The second deposition has a greater purity. In order to increase the yield, high-purity hydrogen is additionally added in the second deposition operation.
However, the process described in DE 1 222 481 B is disadvantageous since two deposition plants in direct series connection are needed, which have to be synchronized. The additional requirement for fresh hydrogen is likewise disadvantageous.
In US 2008/0056979 A1, offgases from a Siemens reactor are introduced into a fluidized bed reactor. The offgases from the fluidized bed reactor can be reprocessed. In this process too, synchronization of the two downstream deposition processes is required.
DE 1 147 567 B discloses a process which reduces the concentration of boron in the polysilicon by suppressing the deposition of boron from BCl3 by means of the law of mass action.
The reaction2BCl3+3H22B+6HClis said to be competing here withSiHCl3+H2Si+3HCl.
Due to the law of mass action, a small HCl concentration shifts the equilibrium to the right, which leads to less boron being deposited.
Any predominance of one or the other of the two competing reactions is additionally influenced by the deposition temperature.
A disadvantage is that the deposition temperature is one of the crucial process parameters in the deposition.
A method such as that according to DE 1 147 567 B would restrict the suitable process windows and make the overall process inflexible.
It was an object of the invention to provide a particularly economically viable process for producing polysilicon, which meets future purity demands.
The object is achieved by a process of the invention.